ES6通过数组中的对象属性求和答案

【问题标题】：ES6 Sum by object property in an arrayES6通过数组中的对象属性求和
【发布时间】：2019-05-14 05:41:20
【问题描述】：

我正在尝试按日期对单位值求和，并创建一个没有重复日期的新数组。例如，我想计算2015-12-04 00:01:00 的总数。该日期在以下数据中出现了 2 次，其值为5 和6，分别为：

[{date: '2015-12-04 00:01:00', unit: 11}, ... etc]

我试过arr = results.map(x => x.unit).reduce((a,c) => a + c)，但它只返回一个值，而不是一个数组。

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
          { unit: 10, date: '2015-12-04 00:01:00' },
          { unit: 5, date: '2015-12-04 00:31:00' },
          { unit: 9, date: '2015-12-04 00:31:00' },
          { unit: 5, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:31:00' },
          { unit: 5, date: '2015-12-04 01:31:00' },
          { unit: 10, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:31:00' },
          { unit: 9, date: '2015-12-04 02:31:00' },
          { unit: 5, date: '2015-12-04 03:01:00' },
          { unit: 9, date: '2015-12-04 03:01:00' },
          { unit: 5, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 04:01:00' },
          { unit: 5, date: '2015-12-04 04:01:00' }];

arr = results.map(x => x.unit).reduce((a,c) => a + c);
console.log(arr);

【问题讨论】：

所以你想总结你拥有的某个日期的数量？
是的，每个日期的总和

标签： javascript arrays dictionary ecmascript-6 reduce

【解决方案1】：

你应该在一个新元素中按日期分组，然后用单位总和创建一个新数组

let indexedDates = {};
results.forEach(result => {
   let arrayByDate = indexedDates[result.date] || [];
   arrayByDate.push(result.unit);
   indexedDates[result.date] = arrayByDate;
});
let yourExpectedResult = Object.keys(indexedDates).map(date => ({
  date,
  unit: indexedDates[date].reduce((a, b) => a +b)
}));

let results = [ { unit: 5, date: '2015-12-04 00:01:00' },
      { unit: 10, date: '2015-12-04 00:01:00' },
      { unit: 5, date: '2015-12-04 00:31:00' },
      { unit: 9, date: '2015-12-04 00:31:00' },
      { unit: 5, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:31:00' },
      { unit: 5, date: '2015-12-04 01:31:00' },
      { unit: 10, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:31:00' },
      { unit: 9, date: '2015-12-04 02:31:00' },
      { unit: 5, date: '2015-12-04 03:01:00' },
      { unit: 9, date: '2015-12-04 03:01:00' },
      { unit: 5, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 04:01:00' },
      { unit: 5, date: '2015-12-04 04:01:00' }];
let indexedDates = {};
results.forEach(result => {
   let arrayByDate = indexedDates[result.date] || [];
   arrayByDate.push(result.unit);
   indexedDates[result.date] = arrayByDate;
});
let yourExpectedResult = Object.keys(indexedDates).map(date => ({
  date,
  unit: indexedDates[date].reduce((a, b) => a +b)
}));
console.log(yourExpectedResult);

【讨论】：

【解决方案2】：

您可以将数据简化为字典，然后将其映射到这样的数组

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
      { unit: 10, date: '2015-12-04 00:01:00' },
      { unit: 5, date: '2015-12-04 00:31:00' },
      { unit: 9, date: '2015-12-04 00:31:00' },
      { unit: 5, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:31:00' },
      { unit: 5, date: '2015-12-04 01:31:00' },
      { unit: 10, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:31:00' },
      { unit: 9, date: '2015-12-04 02:31:00' },
      { unit: 5, date: '2015-12-04 03:01:00' },
      { unit: 9, date: '2015-12-04 03:01:00' },
      { unit: 5, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 04:01:00' },
      { unit: 5, date: '2015-12-04 04:01:00' }]

// first make an object with {date: unit} value as such
const newResults = results.reduce((acc, item) => ({
  ...acc,
  [item.date]: (acc[item.date] || 0) + item.unit
}) , {})

console.log(newResults)
/*
{
  "2015-12-04 00:01:00": 15,
  "2015-12-04 00:31:00": 14,
  "2015-12-04 01:01:00": 15,
  "2015-12-04 01:31:00": 15,
  "2015-12-04 02:01:00": 15,
  "2015-12-04 02:31:00": 14,
  "2015-12-04 03:01:00": 14,
  "2015-12-04 03:31:00": 15,
  "2015-12-04 04:01:00": 15
}
*/

// now if you want array representation for this you can do

const finalResult = Object.keys(newResults).map(key => ({date: key, unit: newResults[key]}))

console.log(finalResult)
/*
[
  {
    "date": "2015-12-04 00:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 00:31:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 01:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 01:31:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 02:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 02:31:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 03:01:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 03:31:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 04:01:00",
    "unit": 15
  }
]
*/

【讨论】：

【解决方案3】：

我想你想要这个？

const x = results.reduce((accu, r) => {
    const { unit, date } = r;
    accu[date] = accu[date] || { unit: 0, date };
    accu[date].unit += unit;
    return accu;
}, {});
const yourNeed = Object.values(x);

【讨论】：

【解决方案4】：

您需要将reduce 中的累加器设为键为日期的对象。然后就可以在units属性中积累了。

您可以使用Object.values() 将其转换回数组。

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
          { unit: 10, date: '2015-12-04 00:01:00' },
          { unit: 5, date: '2015-12-04 00:31:00' },
          { unit: 9, date: '2015-12-04 00:31:00' },
          { unit: 5, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:31:00' },
          { unit: 5, date: '2015-12-04 01:31:00' },
          { unit: 10, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:31:00' },
          { unit: 9, date: '2015-12-04 02:31:00' },
          { unit: 5, date: '2015-12-04 03:01:00' },
          { unit: 9, date: '2015-12-04 03:01:00' },
          { unit: 5, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 04:01:00' },
          { unit: 5, date: '2015-12-04 04:01:00' }];

arr = Object.values(results.reduce((a, {
  unit,
  date
}) => (a[date] = {
  date: date,
  unit: a[date] ? a[date].unit + unit : unit
}, a), {}));
console.log(arr);

【讨论】：